What is the “magnitude” of an earthquake?
Magnitude is a measure of the amount of energy released during an earthquake. It is frequently described using the Richter scale. To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicentre. Since magnitude is representative of the earthquake itself, there is only one magnitude per earthquake. Taking the Saguenay QU earthquake of November 25, 1988 as an example, one could not therefore speak of magnitude 6 at Quebec City and magnitude 4 to 5 at Montreal. The effects (or intensities) experienced at different places were different, but the magnitude of the earthquake is unique; in this example, it was 6 on the Richter scale. Magnitude thus has more to do with the effects of the earthquake overall. The magnitude scale is logarithmic. This means that, at the same distance, an earthquake of magnitude 6 produces vibrations with amplitudes 10 times greater than those from a magnitude 5 ea
